1. Field of the Invention
The present invention relates to a data processing unit, and more particularly relates to means for carrying out a diagnostic check of the data processing unit.
2. Description of the Prior Art
A data processing unit can comprise an LSI (Large Scale Integration) element. The LSI element offers a complete circuit on a very small chip which can contain ten thousand logic circuits or more. It is very difficult for operator to perform a diagnostic check of the LSI data processing unit since it is very difficult to directly test each of the logic circuits contained in the LSI chip by means of a very small testing probe. Furthermore, there are an enormous number of logic circuits contained in the LSI chip, and the chip may be as small as 0.1 square inch in area. Therefore, the diagnostic check is usually carried out by testing the electric signal appearing at each of the external pins extending from the LSI chip. The number of external pins, which is usually about a hundred, is very small when compared to the number of logic circuits contained in a single LSI chip. Accordingly, the operator can easily carry out a diagnostic check of the LSI data processing unit by using a testing probe which is sequentially connected to the respective external pins extending from the LSI chip and thus can detect whether or not the data processing unit is operating correctly. However, if the data processing unit is found to be operating incorrectly during the diagnostic test, it would then be impossible to then utilize this method to further discover and investigate the location of the faults within the chip or the reasons for the errors or troubles occurring within the data processing unit. This is because, as previously mentioned, such a diagnostic check in accordance with the prior art cannot be carried out by directly probing each of the numerous logic circuits contained in the LSI chip. Instead, the diagnostic check is carried out by indirectly probing each of the external pins of the LSI chip.